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A cDNA coding for the human neuronal nicotinic [Alpha]7 receptor subunit with Leu-248 mutated to threonine was expressed in Xenopus oocytes. When activated by acetylcholine (AcCho), the receptors expressed generated currents that had low desensitization, linear current-voltage relation, and high apparent affinity for both AcCho and nicotine. These characteristics are similar to those already described for the chick threonine-for-leucine-247 [Alpha]7 nicotinic AcCho receptor (nAcChoR) mutant (L247T[Alpha]7). These properties were all substantially maintained when the human L248T[Alpha]7 mutant was transiently expressed in human Bosc 23 cells. Simultaneous whole-cell clamp and fluorescence measurements with the [Ca.sup.2+] indicator dye Fura-2 showed that nicotine induced a [Ca.sup.2+] influx in standard 2 mM [Ca.sup.2+] solution. The average fractional [Ca.sup.2+] current flowing through L248T[Alpha]7 nAcChoRs was 6.7%, which is larger than that flowing through muscle [Alpha][Beta][Epsilon][Delta] nAcChoRs (4.1%). The relative [Ca.sup.2+] permeability, determined in oocytes in the absence of [Cl.sup.-], was measured from the shift in reversal potential caused by increasing the external [Ca.sup.2+] concentration from 1 to 10 mM. The human wild-type [Alpha]7 nAcChoR was found to be more permeable than the L248T[Alpha]7 mutant to [Ca.sup.2+]. Our findings indicate that the [Ca.sup.2+] permeability of the homomeric [Alpha]7 nAcChoR is larger than that of the heteromeric neuronal nicotinic receptors studied to date and is possibly similar to that of the N-methyl-D-aspartate subtype of brain glutamate receptors.